1. Technical Field of the Invention
This invention generally relates to the treatment of a subterranean calcareous formation containing crude oil with an acidic composition to aid in the recovery of the crude oil from the formation. The invention more particularly relates to the treatment of the formation with a specific aqueous acidic composition formulated to resist the formation of sludge in the crude oil.
For purposes of this invention, sludge is defined as a solid material formed in crude oil containing asphaltenes and maltenes which constituents may, under certain conditions, precipitate from the crude oil. Formation of sludge in crude oil while the crude oil is in the formation can render very difficult the task of recovery of the oil from the formation. For purposes of this invention crude oil containing quantities of asphaltenes and maltenes subject to the formation of sludge is referred to as heavy crude and sometimes as sludging crude.
2. Description of the Prior Art and Problem Solved
Formation acidizing or, simply, acidizing, is a method well known in the prior art utilized to increase the flow of fluid from a subterranean formation; under the method the formation is contacted with an acidic composition to react with and dissolve materials contained therein for the purpose of increasing the permeability of the formation. The flow of fluid from the formation is therefor increased because of the increase in formation permeability caused by the dissolution of the material. A known method of acidizing a subterranean formation comprises the steps of: conducting an acid composition to the formation through tubing disposed in a borehole penetrating the subterranean formation; forcing the acid composition into contact with the formation and permitting the acid to react with and dissolve certain materials contained therein to thereby enlarge pore spaces within the formation and thus to increase the permeability thereof. Acidizing calcareous formations, such as limestone, has been successfully conducted utilizing hydrochloric acid, certain organic acids, such as acetic acid, citric acid and formic acid, and mixtures thereof.
The object of formation acidizing--increasing formation permeability--can be frustrated if the specific acid employed in the treatment provides an environment which fosters the production of precipitates within the formation which fill and plug the pore spaces in the formation with the consequent result of failing to increase and possibly even decreasing formation permeability. One such undesirable result features the precipitation of ferric iron compounds such as ferric hydroxide from the acid composition used to perform the acidizing. Hydrochloric acid upon spending to ph values of about 2.5 and greater permits the precipitation of ferric hydroxide. The iron precipitated can be introduced into the formation by reaction of hydrochloric acid with iron compounds in the tubing used to conduct the acid to the formation; the iron can be a natural constituent of the formation. Iron control is thus a problem.
Hydrochloric acid may precipitate sludge from the crude oil contacted. The quantity of precipitated sludge is a function of formation temperature, acid concentration, ferric iron concentration and the concentration of asphaltenes and maltenes in the crude oil.
Other acids, such as formic acid and acetic acid, upon spending can cause precipitation of calcium salts if the concentration of the acids are not limited so that the solubility of the salt is not exceeded.
The iron control problem was addressed in U.S. Pat. No. 3,142,335 to Dill, et al., wherein formation acidizing is conducted using an acidic composition of hydrochloric acid and an iron sequestering agent containing a mixture of ingredients such as citric acid or a salt thereof with acetic acid or formic acid or salts thereof.
Crowe, in U.S. Pat. No. 4,317,735, presents an extensive discussion of prior U.S. patents which address the iron control problem. The patents discussed all principally rely on the use of iron sequestering agents mixed with the acidizing medium to prevent the precipitation of iron compounds.
Crowe criticized the use of most prior art sequestering agents because they become ineffective at temperatures in the range of from about 125.degree. to about 150.degree. F.
Crowe further reports that U.S. Pat. No. 4,096,914 teaches that ferric iron reacts with asphaltenic oil to form insoluble iron-asphaltene compounds. It is suggested that incorporation of salicyclic acid into the treatment acid will prevent the formation of such compounds.
Crowe, in U.S. Pat. No. 4,317,735, is particularly concerned that acidizing compositions gelled with aqueous xanthan gums crosslink in the presence of ferric ions upon spending to a ph value in excess of about 1.5 Crowe teaches the solution of this crosslinking problem by including in the gelled acid a soluble alkanoic and/or alkenoic acid additive having at least 4 carbon atoms and bearing at least 2 hydroxyl groups per molecule and/or a soluble salt or gamma-lactone thereof. The additive which includes ascorbic acid, erythorbic acid and their alkali metal salts is said to inhibit the formation or presence of ferric ions in solution and/or the formation of ferric compounds. Crowe comments that the treatment acid utilized together with the xanthan gum and additive can be a mixture of formic acid and acetic acid.